1. Field of the Invention
This invention relates to a ceramic composition for a reoxidation semiconductive ceramic capacitor.
In recent years, the trend in electronic devices has been toward smaller components with high component density and stable quality. Demand has thus increased for electronic components that are compact and of improved quality. Thus, the desirability of developing a reduction-reoxidation type semiconductive ceramic capacitor excelling in insulation resistance and breakdown voltage, enjoying a small size and a large capacity, and yielding only to small dielectric loss has found growing recognition.
2. Prior Art Statement
Heretofore, as materials usable for the reduction-reoxidation type semiconductive ceramic capacitors, the BaTiO.sub.3 -La.sub.2 O.sub.3 -TiO.sub.2 type solid solution composition and the composition having not more than 0.6% by weight of manganese carbonate incorporated in the solid solution mentioned above have been known to the art. The ceramic capacitors formed of these compositions acquire a capacity per unit area of 0.27 .mu.F/cm.sup.2, a dielectric loss expressed tan .delta. of 2.8%, and a breakdown voltage of 800 V as characteristic properties (Japanese Patent Publication No. SHO 51(1976)-44738).
The composition having not more than 1 atomic % of the oxides of Cr, Mn, Fe, Co, and Ni and not more than 2 atomic % of the oxide of Si incorporated in the BaTiO.sub.3 -SrTiO.sub.3 -Ln.sub.2 O.sub.3 -TiO.sub.2 (Ln for a lanthanum series element) type solid solution composition has been also known. The ceramic capacitor formed of this composition acquires a capacity per unit surface area of 0.58 .mu.F/cm.sup.2, a dielectric loss, tan .delta., of 2.8%, and a breakdown voltage of 820 V as characteristic properties (Japanese Patent Public Disclosure No. SHO 58(1983)-14518).
However, there has been desired a capacitor which excels these conventional reduction-reoxidation type semiconductive ceramic capacitors in performance and particularly retains a breakdown voltage of not less than 900 V, possesses a still larger capacity per unit area, and suffers from a still lower dielectric loss. Various studies are now under way for the development of such a capacitor. Generally, a thin dielectric layer in the surface region bears largely on the capacity, insulation resistance, breakdown voltage, and dielectric loss of the reduction-reoxidation type semiconductive ceramic capacity. Though the capacitor's capacity per unit surface area is increased by decreasing the thickness of the dielectric layer, the decrease of the thickness of the dielectric layer inevitably lowers the insulation resistance and the breakdown voltage. Conversely, an attempt to heighten the insulation resistance and the breakdown voltage necessitates an increase in the thickness of the electric layer and entails a decrease in the capacity per unit surface area. While the capacitor's dielectric loss hinges on the tan .delta. of the dielectric layer, it bears in a larger measure on the series resistance component of the boundary between the dielectric layer and the semiconductive ceramic. This series resistance component must be decreased to lower tan .delta.. Particularly since the tan .delta. is an important property for the capacitor, an increase the value of tan .delta. forms a serious drawback. Any decrease in this value, even if slight, is significant. What is required in a new ceramic composition which permits production of a capacitor possessing a high resistivity index and a high breakdown voltage per unit thickness of the dielectric layer and exhibiting a low series resistance component. To be specific, the capacitor formed of this ceramic composition is required to exhibit a breakdown voltage of not less than 900 V, an insulation resistance of not less than 10.sup.10 .OMEGA., and a capacity per unit surface area of 0.60 .mu.F/cm.sup.2 as its characteristic properties.